U.S. Pat. No. 4,560,829 discloses the desirability of foamed fluoropolymer insulation as the electrical insulation in cables to be used over the 3 to 18 GHz frequencies of data transmission. The use of certain blowing agents is disclosed together with melt-extrudable fluoropolymers having a loss tangent less than 0.0015 at 10 GHz is disclosed. The blowing agent is dissolved in the molten fluoropolymer within the extruder, by virtue of the pressure maintained within the extruder. This pressure is released after the molten fluoropolymer is extruded, enabling the dissolved blowing agent to come out of solution within the molten fluoropolymer to thereby form bubbles (voids) within the insulation extruded onto the conductor of the cable. The voids form as the molten polymer is solidifying to lock in the bubbles as cells within the polymer insulation, thereby forming the foamed insulation. The amount of the blowing agent present in the molten polymer is adjusted so as to remain dissolved within the extruder, but not too great so that the foaming of the insulation does not cause blowing out of the bubbles (rupturing) through the exposed surface of the insulation or the surface in contact with the conductor or internally within the thickness of the insulation to create large voids that deteriorate signal transmission performance of the cable. This rupturing is a limitation on the void content achievable in the extrusion foaming step. The melt strength of the fluoropolymer plays a role in this regard, i.e. the higher the melt strength of the fluoropolymer, the greater is the resistance to rupture and the greater is the void content that is achievable.
U.S. Pat. No. 4,764,538 discloses the desirability of dispersing boron nitride and certain inorganic salts into the fluoropolymer to act as a nucleant for the formation of the cells (voids) within the molten polymer forming the foamed insulation, whereby the expansion of the dissolved blowing agent results in the cells formed within the molten polymer being small. The nucleating agent forms the sites for the voids to form. This patent describes the melt draw-down extrusion foaming process, wherein the molten fluoropolymer containing the foam cell nucleating agent is extruded as a tube that is vacuum drawn-down into the shape of a cone, the apex of which is the location of contact with the wire passing through the guide tip of the extruder crosshead. The wall thickness of the cone decreases towards the apex as the faster-running conductor stretches (draws) the cone. The patent discloses that the dissolved gas comes out of solution in the molten fluoropolymer by virtue of the sudden drop in melt pressure as the molten fluoropolymer exits the extrusion die. The evolution of the dissolved blowing agent is delayed until the molten polymer comes into contact with the conductor so as to avoid rupture of the cone caused by the weakening of the melt strength of the molten polymer forming the cone if voids were formed within the cone, especially in its thinning wall approaching the cone apex. The dynamic nature of the foaming process is revealed by Tables II and III in the '538 patent, by foam cell size varying both with the speed of the conductor (line speed) and the length of the cone.
U.S. Pat. No. 4,877,815 discloses an additional improvement in forming foamed insulation, i.e. of smaller cell size and higher void content, using a new class of foam cell nucleating agents, thermally stable sulfonic and phosphonic acids and salts, optionally together with boron nitride and inorganic salt.
EP 0 423 995 discloses another improvement, which is to expose the fluoropolymer to fluorine treatment to react with the unstable end groups of the fluoropolymer to convert them to —CF3 end groups, resulting in a reduction in dissipation factor for the fluoropolymer over the range of 100 MHz to 10 GHz. Table 1 in '995 discloses the effect of transmission frequency on dissipation factor, namely as the frequency increases from 1 MHz to 10 GHz, the dissipation factor advantage of tetrafluoroethylene/perfluoro(propyl vinyl ether) copolymer (TFE/PPVE) over tetrafluoroethylene/hexafluoropropylene copolymer (TFE/HFP) reverses itself, i.e. 0.000087 vs 0.00573 to 0.0010 vs 0.00084, respectively.
U.S. 2008/0283271 (U.S. Pat. No. 7,638,709) discloses improvement in return loss over the transmission frequency range of 800 MHz to 3 GHz by incomplete fluorination of the TFE/HFP copolymer, leaving some of the as-polymerized end groups present in the copolymer, which increases the affinity of the foamed insulation for the conductor. The effect of incomplete fluorination can be obtained by using a single TFE/HFP copolymer, or a mixture of TFE/HFP copolymers, one of which is incompletely fluorinated, which saves the need for making a special single fluoropolymer having the desired melt flow rate. Unfortunately, the dissipation factor of the mixture of TFE/HFP copolymers of Example 1 of '271 is 0.00048, which is too high for such high transmission frequencies as 10 GHz.
There remains a need for foamed insulation that is both economical to manufacture and exhibits good signal transmission properties at high frequencies such as at least 10 GHz.